Various types of non-volatile memory devices employ materials that can be selectively caused to exhibit more than one value of electrical resistivity. To form a single memory cell (i.e., one bit), a volume of such a material may be provided between two electrodes. A selected voltage (or current) may be applied between the electrodes, and the resulting electrical current (or voltage) therebetween will be at least partially a function of the particular value of the electrical resistivity exhibited by the material between the electrodes. A higher electrical resistivity may be used to represent a “1” in binary code, and a lower electrical resistivity may be used to represent a “0” in binary code, or vice versa. By selectively causing the material between the electrodes to exhibit higher and lower values of electrical resistivity, the memory cell can be selectively characterized as exhibiting either a 1 or a 0 value.
One particular type of such non-volatile variable resistance memory devices is the phase change memory device. In a phase change memory device, the materials provided between the electrodes typically are capable of exhibiting at least two microstructural phases or states, each of which exhibits a different value of electrical resistivity. For example, the so-called “phase change material” may be capable of existing in a crystalline phase (i.e., the atoms of the material exhibit relatively long range order) and an amorphous phase (i.e., the atoms of the material do not exhibit any or relatively little long range order). Typically, the amorphous phase is formed by heating at least a portion of the phase change material to a temperature above the melting point thereof, and then allowing the phase change material to rapidly cool, which results in the material solidifying before the atoms thereof can assume any long range order. To transform the phase change material from the amorphous phase to a crystalline phase, the phase change material is typically heated to an elevated temperature below the melting point, but above a crystallization temperature, for a time sufficient to allow the atoms of the material to assume the relatively long range order associated with the crystalline phase.
For example, Ge2Sb2Te5 (often referred to as “GST”) is often used as a phase change material. This material has a melting point of about 620° C., and is capable of existing in amorphous and crystalline states. To form the amorphous (high resistivity) phase, a portion of the material is heated to a temperature above the melting point thereof by passing a current through the material between the electrodes and heating the material (the heat being generated due to the electrical resistance of the phase change material) for as little as 10 to 100 nanoseconds. As the GST material quickly cools when the current is interrupted, the atoms of the GST do not have sufficient time to form an ordered crystalline state, and the amorphous phase of the GST material is formed. To form the crystalline (low resistivity) phase, a portion of the material may be heated to a temperature of about 550° C., which is above the crystallization temperature and near, but below, the melting point of the GST material, by passing a lower current (lower than the current used in forming the amorphous phase, as described above) through the GST material between the electrodes to heat the GST material (to a temperature above the crystallization temperature but below the melting point) for an amount of time (e.g., as little as about 30 nanoseconds) to allow the atoms of the GST material to assume the long range order associated with the crystalline phase, after which the current flowing through the material may be interrupted. One of the melting current (the current passed through the phase change material to form the amorphous phase) and the crystallization current (the current passed through the phase change material to form the crystalline phase) may be referred to as the “write current,” and the other of the melting current and the crystallization current may be referred to as the “reset current.” The write current and the reset current may be collectively referred to as the “programming currents.”
Various memory devices having memory cells comprising variable resistance material, as well as methods of forming and using such memory devices are known in the art. For example, memory cells comprising variable resistance materials and methods of forming such memory cells are disclosed in U.S. Pat. No. 6,150,253 to Doan et al. (issued Nov. 21, 2000), U.S. Pat. No. 6,294,452 to Doan et al. (issued Sep. 25, 2001), United States Patent Application Publication No. 2006/0034116 A1 to Lam et al. (published Feb. 16, 2006), U.S. Pat. No. 7,057,923 to Furkay et al. (issued Jun. 6, 2006), United States Patent Application Publication No. 2006/0138393 A1 to Seo et al. (published Jun. 29, 2006), and Unites Stated Patent Application Publication No. 2006/0152186 A1 to Suh et al. (published Jul. 13, 2006). Furthermore, supporting circuitry that may be used to form a memory device comprising memory cells having a variable resistance material, as well as methods of operating such memory devices, are disclosed in, for example, United States Patent Application Publication No. 2005/0041464 A1 to Cho et al. (published Feb. 24, 2005), U.S. Pat. No. 7,050,328 to Khouri et al. (issued May 23, 2006), and U.S. Pat. No. 7,130,214 to Lee (issued Oct. 31, 2006).
The high amounts of energy required to heat a volume of phase change material for the programming operations (e.g., writing and reset operations) in phase change memory devices has hindered their widespread implementation in the memory device market. Thus, there is a need in the art for methods, structures, and devices for decreasing the required programming energy (i.e., current) in phase change memory devices and systems.